Crystals shatter ideas about Earth’s history

By By Robert Ireland

By Robert Ireland

The age of the Earth can be divined with crystals, scientists say.

Not crystal balls, but ancient crystals, smaller than the diameter of a human hair and predating the oldest rocks ever found.

The crystals recorded surprising evidence of the climatic conditions on early Earth. This evidence is prompting geologists to revise a long-held theory on the construction of the earth.

“Geologists study rocks; they are our books,” said John Valley, professor of geology and geophysics at the University of Wisconsin-Madison.

Valley explained his research for a Frontiers of Science lecture March 8 in the biology building.

Zircon crystals found in the Jack Hills of western Australia suggest the presence of continents and oceans on a cool early Earth where the conditions favorable for the evolution of life presumably existed much earlier than once supposed.

Radioactive uranium impurities trapped within the crystalline structure of the mineral zircon are like a ticking geologic clock that permit researchers to accurately measure the extreme age of crystals, Valley said.

By analyzing rocks and studying the strata where they occur, geologists read lingering clues of the processes and conditions under which they formed.

Planetary scientists believed that a barrage of meteors bombarded the Earth’s surface during the first 500 million years of the its 4.5-billion-year existence.

“The Earth was a fiery ball covered with magma oceans and an acidic atmosphere,” Valley said.

No rocks have ever been recovered from this hellish period, aptly named the Hadean Period. The traditional view held that planetary crust and continents could not form until the Earth cooled at the end of the Hadean.

With no surviving evidence, Valley described the Hadean as “what we would call geologically, the dark ages.”

Researchers previously dated zircons from the Jack Hills at fewer than 4.3 billion years old.

Taking advantage of advances in ion microprobe technology, Valley and fellow researchers pushed the age of the oldest zircons back to 4.4 billion years.

They then found something unexpected when using new techniques to measure the ratio of oxygen isotopes within the oldest crystals.

Oxygen occurs on Earth as two different stable isotopes. Most oxygen has an atomic weight of 16, but there is a minute amount of oxygen with an atomic weight of 18. Zircons formed deep within the Earth’s mantle record an oxygen isotope ratio different from that of zircons formed from rocks in Earth’s crust.

When crustal rocks get buried, melt and then re-crystallize, the reformed zircon crystals within the rock assume an oxygen isotope value with a higher 18-to-16 atomic-weight ratio, which indicates exposure to liquid water.

Valley expected that his zircons would bear an isotopic signature of the Hadean Period, similar to zircons formed in the hot mantle. Instead, he found a ratio comparable to younger zircons formed from crustal rocks on a wet and cool Earth.

“When we first found these high [oxygen 18] readings, I thought it was a mistake,” Valley said.

For more than a year, Valley and his team delayed publishing the results to verify their findings. Meanwhile, other researchers obtained similar results and the antiquity of the zircons held up to scrutiny.

Valley and his team were astonished. Valley described the Jack Hills zircons as “tiny time capsules” that indicate that Hadean conditions on the primitive Earth ended as much as 400 million years earlier than previously thought.

“It wasn’t something we set out to find,” he said.

Barbara Nash, U professor of geology and geophysics, said she regards Valley’s work as a remarkable achievement.

“The determination that the process involved water is justified,” she said. “Nobody has proposed an argument equally as compelling.”

Erin Peterson, a senior in political science and public relations who said she grew up on the Discovery Channel, appreciated the “intrinsic value” of Valley’s research.

“It encourages us to challenge what we believe and what we’ve been taught,” said Peterson.